6 comments:

A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the aperture (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.) Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape. The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using horizontal boring mills. Circular covers do not need to be rotated to align them when covering a circular manhole. Human beings have a roughly circular cross-section. A round manhole cover can be more easily moved by being rolled. If an automobile rolls over a dislodged manhole of another shape, the sharp corners could puncture the automobile's tire, whereas a circle doesn't have corners. It's easier to dig a circular hole. Tradition Aesthetics Other manhole shapes can be found, usually squares or rectangles. Nashua, New Hampshire may be unique in the U.S. for having triangular manhole covers that point in the direction of the underlying flow. The city is phasing out the triangles, which were made by a local foundry, because they are not large enough to meet modern safety standards, and larger triangles cannot be foundLotsa reasons, which one do you prefer?

Round tubes are the strongest and most material-efficient shape against the compression of the earth around them. The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic.

Round castings are much easier to machine using horizontal boring mills. Circular covers can also be moved around by rolling, and they need not be aligned to put them back.